1. Field of the Invention
This invention relates to separators to be used in secondary lithium batteries.
2. Description of the Prior Art
A secondary lithium battery fundamentally consists of an anode, a cathode and a separator located between these electrodes. This separator contains an electrolyte solution in pores of a separator material, which enables ions liberated from the electrode materials to migrate between the electrodes during the charge/discharge cycle.
Known examples of separator materials for secondary lithium batteries include glass fiber fabrics, microporous polyolefin membranes and non-woven fabrics, and a microporous material obtained by extracting with a solvent a mixture of a vinylidene fluoride-hexafluoropropylene copolymer (hereinafter sometimes referred to as (VdF-HFP) copolymer resin) and a plasticizer so as to eliminate the plasticizer from the mixture and then making the residue microporous as disclosed in National Publication No. 8-509100 of the Japanese Translation of International Publication WO95/06332 (PCT/US94/08772) and National Publication No. 9-500485 of the Japanese Translation of International Publication WO95/15589 (PCT/US94/12641).
Regarding secondary lithium batteries using separator materials of the latter type, such a separator comprises a (VdF-HFP) copolymer resin employed as the base material, silica employed as a filler and dibutyl phthalate employed as a plasticizer. This separator material is inserted between a fluorinated polymer-containing cathode membrane and an anode membrane to give a three-layered laminate. Next, the above plasticizer is extracted with a solvent to thereby form pores having electrolyte sustained therein. Owing to this constitution, the battery can achieve a remarkably elevated ion conductivity compared with conventional secondary lithium batteries of the polymer (solid electrolyte) type. Namely, the battery of this type can give an ion conductivity comparable to those of liquid-type secondary lithium batteries.
When the above-mentioned separator material having a (VdF-HFP) copolymer resin as a base resin is laminated onto the a electrodes, however, there arises a problem of short-circuiting. The present inventor studied factors causative of this short-circuiting and, as a result, estimated that short-circuiting would be induced as follows. In the step of the lamination, the electrode membranes and separator are softened by heating. In practice, not only the interfaces thereof but also the whole electrode membranes and separator are softened. When pressure for adhering the interfaces is applied in this state, active material particles having electrically conductive carbon in the electrode membranes adhering thereto penetrate into the softened separator membrane, thus causing short-circuiting.